1. Field of the Invention
The present invention relates to a method of producing fullerene-dispersed ceramics in which fullerene is effectively compounded into ceramics.
2. Description of Related Art
As known in the art, fullerene-dispersed ceramics can be advantageously applied to various materials, such as self-lubricating abrasive materials, low-friction, abrasion-resistance materials, ferroelectric or piezoelectric materials having high permittivity due to interface-polarization effect, computer memory, materials micro-actuator materials and the like. Therefore, fullerene-dispersed ceramics can be used in a wide range of industrial fields such as automobiles, domestic electric appliances, computers and the like.
The inventors have already proposed a method for producing a composite material of ceramics and fullerene such as C.sub.60, as disclosed in Kunichi Miyazawa et al., "Characterization of C.sub.60 -Doped Zirconia Prepared from Organic Solutions", Processing and Fabrication of Advanced Materials VI, Volume I, The Institute of Materials, 1998, pp. 775-784. According to the method disclosed in this paper, the composite material is formed by preparing a solution of C.sub.60 and metal alkoxide in which an organic solvent such as toluene is applied, and subjecting the solvent to drying and baking so as to make C.sub.60 -ceramics composite powder, and sintering the composite powder while it is filled in a copper case.
FIG. 1 shows a flowchart of the production process of the fullerene-dispersed ceramics according to the above-mentioned proposal, in which toluene is filled in a glass beaker in an amount of 100 to 200 ml. Zirconium tetra n-propoxide (ZNP) is added into the beaker and dissolved while stirring the solution. Subsequently, C.sub.60 powder is added into the beaker and dissolved while stirring the solution. The toluene solution with C.sub.60 and ZNP dissolved therein is left standing for about 1 to 3 days to cause gelation of the solution. The precipitation resulting from the gelation is dried in a vacuum atmosphere at a temperature of about 100 to 110.degree. C., so as to obtain a dry gel of C.sub.60 comprised of ZNP. The dry gel is baked to form a zirconia composite powder comprised of C.sub.60, in an air atmosphere at, 400.degree. C. for 30 minutes. In forming a sintered body, the powder is filled into a copper case in a nitrogen or an argon atmosphere, and then sintered at a temperature of 600.degree. C. to 900.degree. C., for 2 hours. The composition of the C.sub.60 -doped zirconia is, for instance, ZrO.sub.2 -3 to 30 mass% C.sub.60.
With the above-mentioned method, however, it was often difficult to obtain composite ceramics in which C.sub.60 was uniformly dispersed. FIG. 2a shows a scanning electron microscope image of the C.sub.60 -dispersed zirconia ceramics sintered body produced by the method shown in FIG. 1. FIG. 2b shows a carbon X-ray image of the sami specimen at the same site as shown in FIG. 2a. It is clear from the FIG. 2b that carbon is segregated in a size of 10 .mu.m, and is not uniformly dispersed in the ceramic matrix.
In order to improve the uniformity of C.sub.60 dispersion in the ceramic matrix, the inventors developed a method of producing a composite material of ceramics and fullerene by utilizing a so-called micelle formation method. According to this method, C.sub.60 -dispersed zirconiapowder is produced by forming C.sub.60 micelle from a water solution of zirconyl nitrate (ZrO(NO.sub.3).sub.2) wherein C.sub.60 is added, by using C.sub.16 TMA (CH.sub.16 H.sub.33 N(CH.sub.3).sub.3 Cl) as a surfactant, followed by formation of zircoma-gel corprising C.sub.60 by controlling the acidity of the solution, and subsequent drying of the zirconia gel. The method is disclosed in Kunichi Miyazawa et al., "Production of C.sub.60 -ZrO.sub.2 composite material and structural evaluation thereof" Abstracts of the Japan Institute of Metals, 1998, pp. 208. In this paper, C.sub.60 -dispersed zirconia powder is produced by dissolving C.sub.16 TMA in a water solution of 100 cm.sup.3 containing 0.1 mole ZrO(NO.sub.3).sub.2, adding C.sub.60 into the solution, stirring the solution for 12 hours, filtering the precipitate formed by dropping 2 mole NaOH in the solution, washing the precipitate by distilled water and then drying in the conditions of 150.degree. C., 8 hours. According to this method, C.sub.60 can be uniformly dispersed in the ceramics matrix, though it has been found that carbon other than C.sub.60 still remains in the matrix. It would be highly desirable to realize C.sub.60 -dispersed ceramics, having a further improved purity.